by Robert R. McMath
Written: February, 1941.
Printed in Reflections: January 1998.

[This is the second and final segment of the article on the McMath 24 inch
telescope printed
with permission from the Bentley Historical Library - Ed; See also
the first segment of this article]

Camera And Guiding Eye Piece Assembly

Plate 5

The camera end of the new 24” reflector, Plate 5, represents a rather
unique departure from
standard telescope design. Since this instrument was intended primarily
for work on the lunar
and planetary programs of the observatory, the 35 mm camera was used as
a nucleus about which
the driving and guiding accessories were constructed. This general
design procedure has
produced a dual guiding system which embodies all the features deemed most
desirable by the
observing staff based on several years experience experience with our particular
photographic problem. The lower guiding assembly, intended for lunar
or planetary work, guides
directly on the central cone of light being photographed. This is
accomplished by the use of
the rotating shutter and mirror combination shown in the enlarged print
of the camera support
casting (Plate 6). S is the shutter disk carrying a semi-circular
plane mirror R and the easily
interchanged shutter plate T. U is the last step in the Selsyn driven gear
train which operates
the camera and shutter. At V is shown the small counterweight balancing
the entire shutter
shaft assembly.

Plate 6

During the guiding period the light beam from the secondary is reflected
through 90 deg. by
the shutter mirror R and comes to a focus on the illuminated reticule.
The optical center of
this reticule can easily be shifted by means of the adjusting screws to
any chosen guiding
point in the field. The reticule can then be rotated so that the
guide point will follow the
wires in both right ascension and declination. Extension tube G includes
a 65 mm lens set for
approximately conjugate foci to transfer the image from the reticle through
the rotatable right
angle prism F to the guiding eyepiece E.

Throughout the open-shutter or photographing period, the mirror lies outside
of the central
light cone and therefore reflects no image through to the guiding eyepiece.
This important
feature prevents the observer from shifting the telescope
while a photograph is being taken. Also, since the reticle to mirror and
camera focal plane to mirror distances were set accurately in our shop,
the telescope
can be focused at the guiding eye piece through use of this guiding system.

By shifting plate K against the stop plate O, the low power composition
eye piece H is brought
into the optical axis position. Here, by means of a focal plane diaphragm
which is a duplicate
of the camera aperture, the observer is able to view the exact field which
will be photographed
at the camera. This same setting can also be obtained on a ground
glass placed at the camera
focal plane, but this latter procedure requires the removal of the camera
from its mounting.

The second guiding system is intended primarily for lunar and stellar work.
The guiding eye
piece assembly--made up of the illuminated reticule, transfer lens, right
angle prism, and
guiding eye piece previously described--is slipped out of its bayonet lock
on the plate K and
moved to a new position at D. Flange D is part of an adjustable tube which
carries a small
reflecting prism at its opposite end. This prism, together with the
eye piece assembly, can be
moved radially, by means of the adjusting screw, from the outer edge of
the cone of light
coming through the central hole in the primary, to the edge of the inner
central cone being
photographed. The entire assembly can also be revolved about the
optical axis, by use of coarse
and fine setting motions, through 360 deg to insure a large field from
which to choose a
guiding point. Obviously, because of this set-up, the stellar images
near the outer edge of the
light cone are not perfectly sharp, but are of sufficiently high quality
to permit their use as
guiding reference points.

The new 35 mm camera is shown at L, along with camera driving Selsyn at
P, primary mirror back
support at C. and secondary mirror focusing handle at A.

An electrical contactor to record exposure times on the chronograph in
the control room is
mounted on the camera support casting back plate, and cannot be seen in
this photograph. One
additional new accessory, a comparison type photometer to be used as an
exposure meter, is not
complete at the present time.

Conclusion

In conclusion, it may be well to add a few words in regard to the program
ahead of the new
instrument. We had long discussed amongst ourselves the desirability
of doing over the
original lunar planetary educational motion picture films with an instrument
with more
resolution and better mechanical adjuncts. The first few months’ use of the
new telescope convinces us that we can do very much better work with this
new instrument
than with the old.

We have already secured several hundred feet of 35 mm motion picture negative
of the disk of
Jupiter. Using the 100-foot E F L combination, the disk of Jupiter
is approximately 7 mm in
diameter, and a shadow transit of one of the satellites has been clearly
observed on one of
the above films. We have been hoping for a blue sensitive emulsion
with sufficient speed and
lack of grain so that enlarged pictures of Jupiter on the screen will not
be too grainy. Just
recently Dr. C. J. Staud of the Eastman Research Laboratories has sent
us some very promising
material for this purpose. After our early experiments with Jupiter,
we plan to
photograph Saturn in the same way and hope to record some activity on the
disk of the planet.

Our experimental lunar pictures showing the sunrise and sunset are very
promising, and it is
our expectation that within a year or so we can make up educational reels
for distribution to
institutions desiring them.

Mr. George H. Malesky of our staff collaborated with the writer in the
design of this new
instrument, and all drawings were made by Malesky. To our instrument
maker, Mr. C. W. Guenther,
we owe thanks for successfully making the difficult shutter assembly.
Dr. Heber D. Curtis gave
us valuable suggestions in regard to the mounting of the primary mirror.
And we are greatly
indebted to Messrs. Sawyer, Mohler, and Brodie of the staff for their work
in the installation
and adjustment of the telescope.

The entire staff of the McMath-Hulbert Observatory of the University of
Michigan is very
grateful to the donors for making possible the acquisition of this new
addition to the instrumentation of the observatory.